The present disclosure relates to ultrasonic imaging. More particularly, the invention relates to a system and method for fundamental imaging of the non-linear response of tissue perfused with a contrast agent.
Ultrasonic imaging has quickly replaced conventional X-rays in many clinical applications because of image quality, safety, and low cost. Ultrasonic images are typically formed through the use of phased or linear array transducers which are capable of transmitting and receiving pressure waves directed into a medium, such as the human body. Such transducers normally comprise multielement piezoelectric materials, which vibrate in response to an applied voltage to produce the desired pressure waves.
To obtain high quality images, the transducer is constructed so as to produce specified frequencies of pressure waves. Generally speaking, low frequency pressure waves provide deep penetration into the medium (e.g., the body), but produce poor resolution images due to the length of the transmitted wavelengths. On the other hand, high frequency pressure waves provide high resolution, but with poor penetration. Accordingly, the selection of a transmitting frequency has involved balancing resolution and penetration concerns. Unfortunately, resolution has suffered at the expense of deeper penetration and vice versa. Traditionally, the frequency selection problem has been addressed by selecting the highest imaging frequency (i.e., best resolution) which offers adequate penetration for a given application. For example, in adult cardiac imaging, frequencies in the 2 MHz to 3 MHz range are typically selected in order to penetrate the chest wall. Lower frequencies have not been used due to the lack of sufficient image resolution. Higher frequencies are often used for radiology and vascular applications where fine resolution is required to image small lesions and arteries affected by stenotic obstructions.
Recently, new methods have been studied in an effort to obtain both high resolution and deep penetration. One such method is known as xe2x80x9charmonic imaging.xe2x80x9dHarmonic imaging is grounded on the phenomenon that objects, such as human tissues, develop and return their own non-fundamental frequencies, i.e., harmonics of the fundamental frequency. This phenomenon and increased image processing capabilities of digital technology, make it is possible to excite an object to be imaged by transmitting at a low (and therefore deeply penetrating) fundamental frequency (ƒo) and receiving reflections at a higher frequency harmonic (e.g., 2ƒo) to form a high resolution image of an object. By way of example, a wave having a frequency less than 2 MHz can be transmitted into the human body and one or more harmonic waves having frequencies greater than 3 MHz can be received to form the image. By imaging in this manner, deep penetration can be achieved without a concomitant loss of image resolution.
Harmonic imaging can be particularly effective when used in conjunction with contrast agents. In contrast agent imaging, gas or fluid filled micro-sphere contrast agents are typically injected into a medium, normally the bloodstream. Because of their strong non-linear response characteristics when insonified at particular frequencies, contrast agent resonation can be easily detected by an ultrasound transducer. More specifically, a second harmonic response occurs when a contrast agent under ultrasonic pressure xe2x80x9cmapsxe2x80x9d energy into the harmonics of the incident pressure waves, instead of the fundamental. Various non-linear detection schemes take advantage of the fact that contrast agents produce non-linear responses of a greater magnitude than the surrounding tissue. By using harmonic imaging after introducing contrast agents, medical personnel can enhance imaging capability for diagnosing the health of blood-filled tissues and blood flow dynamics within a patient""s arterial system. For example, contrast agent harmonic imaging is especially effective in detecting myocardial boundaries, assessing microvascular blood flow, and detecting myocardial perfusion. Transducers have been designed for transmit frequencies in the range of 2 MHz to 3 MHz for sufficient resolution of cardiac valves, endocardial borders and other cardiac structures.
The power or mechanical index of the impinging ultrasound signal directly affects the contrast agent acoustical response. At lower powers, microbubbles resonate and emit harmonics of the transmitted frequency. The magnitude of these microbubble harmonics depends on the magnitude of the excitation signal pulse. At higher acoustical powers, microbubbles rupture and emit strong broadband signals.
A prior art diagnostic system, disclosed by Johnson et al. in U.S. Pat. No. 5,456,257, teaches improved imaging by introducing coated microbubble contrast agents in the body of a patient. The ""257 patent further teaches destroying the contrast agents with ultrasonic energy and detecting the microbubble destruction through phase insensitive detection and differentiation of echoes received from consecutive ultrasonic transmissions.
The present invention relates to a system and method for real-time imaging of tissue perfused with a contrast agent. An image with increased sensitivity to non-linear responses, can be achieved by detecting the ultrasound response at the fundamental frequency from tissue perfused with a contrast agent and excited by multiple excitation levels. Briefly described, in architecture, an ultrasonic contrast agent and tissue imaging system can be implemented with a transducer, first and second transmitters, a receiver, a control system, a processing system and a display. The ultrasonic contrast agent and tissue imaging system may be configured such that the first and second transmitters generate first and second electrical signals that are translated by the transducer into first and second pressure waves respectively, the respective pressure waves being of different magnitudes. A control system electrically coupled to the first and second transmitters, the transducer, and a receiver coordinates pressure wave transmissions and the reception of ultrasonic responses from the insonified contrast agent and tissue. A processing system electrically coupled to the receiver processes the multiple responses in a manner such that linear responses are removed leaving the non-linear responses from the contrast agent and the surrounding tissue. Lastly, a display configured to receive the non-linear response creates an image of the insonified contrast agent and surrounding tissue.
The present invention can also be viewed as providing one or more methods for non-linear ultrasonic response signal detection. In this regard, one such method can be broadly summarized by the following steps: introducing a contrast agent into the tissue targeted for imaging; insonifying the tissue at a first intensity to generate a first response; insonifying the tissue at a second intensity, wherein the second intensity is different from the first, to generate a second response; separately measuring the first and second responses at the fundamental frequency; generating a projected response from the first response; and mathematically manipulating the projected response and the second response to derive the non-linear response.
Other systems, methods, features, and advantages of the present invention will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.